A conventional window shade generally is lifted or lowered through a rope. The rope is exposed outside and could cause a hazardous condition by tangling or strangling children playing nearby. Hence for safety reason, the rope of the window shade is generally collected in a winding device located on the bottom rail of the window shade without exposing to prevent risks.
There are many types of pulling cord winding devices for window shades on the market. FIG. 1 illustrates one of the examples (namely U.S. Pat. No. 6,024,154). It has a pulling cord winding device A installed on a bottom rail B1. The device includes a cord braking board A1 which has two S-type winding sets C installed on two side. The S-type winding sets C are mounted onto two corresponding winding struts C1 and C2. The winding strut C1 located on the inner side is engaged with a gear A11 which in turn is engaged with a gear rack A10. Engagement between the gear A11 and the gear rack A10 could go awry and result in slipping of the gear A11 caused by pulling of a pulling cord C3 while the window shade B is extended or retracted. Moreover, the crest A110 of the gear A11 is prone to be hit by the gear rack A10 on a specific spot and result in wearing (as shown in FIG. 2). This could make anchoring of the pulling cord C3 difficult. Furthermore, because the pulling cord winding device A contains two S-type winding sets C on the two sides of the cord braking board A1, it has too many elements and becomes too bulky. The bottom rail B1 must have a bigger size to accommodate the device. In short, the entire device takes too much space and is too costly. The enlarged bottom rail B1 also becomes too heavy. And the spring installed inside the pulling cord winding device A has to withstand a greater force. This shortens the service life of the device.
FIG. 3 shows another conventional pulling cord winding device disclosed in U.S. Pat. No. 6,575,223 B1. The winding device E is installed in a bottom rail D1 of a window shade D. It includes a cord braking board E1 which has two corresponding cams E2 on two sides. Each cam has latch troughs E20. A pressing plate E3 is located between the two cams E2 that has two latch arms E4 to latch on the cams E2 (referring to FIG. 4A). When the window shade D is extended or retracted, the latch arms E4 tend to slide under the impact of external forces. As a result, coupling between the latch arms E4 and the latch troughs E20 is not secured. Hence the window shade D is difficult to be positioned and anchored as desired during extension or retraction (referring to FIG. 4B).